In general, an electrophotographic photosensitive member, in particular, an electrophotographic photosensitive member using an organic material (organic photosensitive member), includes a supporting member and at least one layer formed by coating (coating film) on the supporting member.
A typical coating process used in manufacturing the electrophotographic photosensitive member includes immersing a member to be coated (supporting member or a supporting member with at least one layer formed thereon) in a coating solution in a coating vessel and lifting the member to be coated so that the coating solution adheres on the surface of the member to be coated and thereby forms a coating film. For immersion and lift, a holder member for holding the member to be coated and a lift for moving the member to be coated held by the holder member up and down are used.
The thickness of the coating film formed by a dip-coating process is basically determined by the viscosity of the coating solution, the volatility of the solvent in the coating solution (coating film), the rate of lifting the member to be coated, etc. The coating film formed on the surface of the member to be coated is initially in a wet state and sags downward in the direction of gravitational force until a particular amount or more of the solvent in the coating film evaporates and the coating film becomes substantially dry. As a result, the thickness of the coating film at the same position undergoes changes immediately after lift.
When the coating film is affected by ambient wind during evaporation of the solvent, the degree at which evaporation proceeds varies locally, and the degree of sagging of the coating film becomes nonuniform, resulting in uneven coating film thickness. This is because when the solvent evaporates from the coating film under ambient wind into solvent vapor, a bias is generated in the concentration of the solvent vapor around the coating film due to the local differences in the degree at which the evaporation proceeds.
Another example of the phenomenon causing the unevenness in the coating film thickness other than the sagging of the coating film in the direction of gravitational force is a phenomenon in which the coating solution adhering on the surface of the member to be coated moves in a particular direction irrelevant to the direction of gravitational force in a biased manner due to actions such as surface tension, intermolecular force in the coating solution, etc.
When the thickness distribution is locally nonuniform due to the various phenomena described above, i.e., when there is a thickness variation, image formation using an electrophotographic photosensitive member is adversely affected.
A popular and effective approach for preventing the thickness variation in the coating film is to lift the member to be coated while covering the side surface of the member to be coated with a hood. When the hood is used during evaporation of the solvent from the coating film in a wet state, the local difference in the degree at which the evaporation proceeds induced by ambient wind can be suppressed.
Another proposed approach is to use a hood formed by connecting a plurality of tubular members such that the hood is extendable and retractable by sliding the respective tubular members (also known as telescopic sliding hood).
Japanese Patent Laid-Open No. 07-104488 teaches a method in which a member to be coated is immersed in a coating solution in a coating vessel and lifted while covering the side surface by extending and retracting the telescopic sliding hood in association with the lift operation.
Japanese Patent Laid-Open No. 63-007873 teaches a coating method in which an telescopic sliding hood is used and the vapor of the solvent evaporating from the coating solution is discharged outside the telescopic sliding hood so that the solvent vapor concentration is low around the coating film on the member to be coated. According to this method, since the solvent vapor concentration around the coating film is low, the time required for evaporation of the solvent can be shortened, and various phenomena occurring during solvent evaporation can be suppressed.
Electrophotographic apparatuses are now being required to achieve higher performance, in particular, higher sensitivity and higher image uniformity. To meet such a requirement, further thickness reduction of the coating film is desirable. When the thickness is reduced, the effect of the thickness variation on the quality of the electrophotographic apparatus becomes greater.
Under such circumstances, the technique of lift the member to be coated while covering the side surface of the member to be coated with the telescopic sliding hood or the technique of evacuating the solvent vapor inside the telescopic sliding hood to outside thereof is no longer sufficient. In other words, a solvent evaporation environment more stable than that in the related art is desired.
Patent Citation 1
Japanese Patent Laid-Open No. 07-104488
Patent Citation 2
Japanese Patent Laid-Open No. 63-007873